Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

Abstract

Due to the lowest‐cost and best reliability, CdTe solar cells are the leading thin‐film photovoltaic technology. Increasing open‐circuit voltage by reducing recombination represents the most promising path toward further improvements. Analysis is needed to identify limitations that cause efficiency losses. To achieve this goal for Cu‐doped CdSe/CdTe solar cells, time‐resolved spectroscopy and microscopy are developed and applied. Recombination lifetimes and radiative efficiency identify that defect‐mediated recombination is the dominant voltage loss mechanism. When carrier lifetimes are averaged over many crystalline grains, they increase from 180 to 430 ns when Al2O3 is applied to the back contact. The quasi‐Fermi‐level splitting correspondingly increases from 880–905 to 906–931 mV, indicating a pathway to overcome the long‐standing 900 mV voltage limitation. However, the dominant recombination losses are attributed to the absorber bulk. From microscopic carrier lifetime measurements, it is identified that space charge fields due to charged grain boundaries (GBs) lead to recombination in the CdTe absorber bulk. At high injection, GB space charge fields are screened, but that occurs above 1 Sun excitation conditions. Alloying with selenium in the near‐interface CdSeTe absorber region reduces GB losses and is identified as one of the factors leading to high radiative and power conversion efficiency.

Charge Carrier Lifetime Fluctuations and Performance Evaluation of Cu(In,Ga)Se2 Absorbers via Time‐Resolved‐Photoluminescence Microscopy

“…In addition, the voltage loss can also be quantified from doping concentration, and using the lifetime data to determine the minority carrier concentration of the material, as shown elsewhere. [38,39] Similar results are obtained using the calibration constant or minority carrier concentrations from lifetime data in most cases (see Section S6, Table S1, Supporting Information), disregarding NaF sample. The disagreement between these two methods in NaF sample is explained by the overestimation of doping concentration from C-V data, as detailed below.…”

Band Bending at CdTe Solar Cell Contacts: Correlating Electro‐Optical and X‐Ray Photoelectron Spectroscopy Analyses of Thin Film Solar Cells

“…The emission spectrum is broad and distinctly different from PL emission of Cu-doped CdTe solar cells. [16,17] Such spectral features are introduced by As dopants [18] and indicate electrostatic potential fluctuations. [19] In comparison to the absorber without passivation (sample 1), emission intensity is higher for sample 2 with HTL.…”

Understanding and Advancing Bifacial Thin Film Solar Cells under Dual Illumination

“…This condition was investigated because there is no simple experimental method to isolate the band bending and S B , [ 4,14 ] and most measurements simply report an S eff . [ 15–17 ]…”